Although electronic cigarettes (e-cigarettes) have been on the market for less than a decade, use has been doubling each year since 2008, and sales are projected to reach $2.2 billion in 2014. Moreover, the traditional tobacco industry has moved into this market, projecting greater growth in the future. Research on e- cigarettes is still in the nascent stage, with open questions about their health consequences, addiction liability, and potential as a smoking cessation tool. E-cigarettes are not regulated-and therefore cannot be overtly marketed-as pharmacotherapy for treating tobacco dependence, yet survey research has found that the primary motivations for their use are to aid smoking cessation or smoking reduction. Although millions of tobacco smokers are simultaneously using e-cigarettes (dual users), often for the purpose of quitting smoking, they usually do so without receiving any behavioral assistance. Despite users' intentions, dual use may maintain tobacco dependence by allowing nicotine delivery when smoking is prohibited, while simultaneously exposing dual users to additional, as-yet-unknown health risks associated with chronic e-cigarette use. The opportunity exists to assist the large population of dual users-smokers who have already demonstrated initial action toward smoking cessation by commencing use of e-cigarettes-with smoking cessation and eventual e- cigarette cessation. This naturally-occurring, self-selected, and motivated group may be ideally primed to benefit from a minimal intervention that may enhance their chances of achieving and maintaining tobacco and nicotine abstinence. The primary aims of the present application are: (1) to adapt via a systematic, formative process a validated self-help, smoking-cessation intervention to meet the needs of current dual users; (2) to test this new intervention in a randomized controlled trial (RCT) of 2500 dual users; and (3) to calculate the cost- effectiveness of the intervention. Our previous research has demonstrated that a series of self-help booklets can enhance smoking cessation and reduce relapse. We will adapt this intervention for dual users. In the RCT, the new intervention will be compared to both no-treatment (assessment-only) and existing, non-targeted smoking cessation manuals. Follow-up will occur through 24 months post-enrollment, and outcomes will include both cigarette and e-cigarette cessation. A secondary aim is to gather longitudinal data regarding the patterns of tobacco and e-cigarette use and the course of cessation of either product among dual users. This project tests whether the high-risk behavior of dual use can be capitalized upon to facilitate smoking cessation and, ultimately, cessation of all nicotine products.